This invention relates to a braided tubular support for a film of polymer which functions as an asymmetric semipermeable membrane in microfiltration (MF) and ultrafiltration (UF) applications. The braided tube is no more than about 3 mm in outside diameter and relies on the polymer film to imbue the fiber membrane product with sustainable high flux along with sufficient abrasion resistance such that a skein of fibers (also referred to as a xe2x80x9cmodulexe2x80x9d) can operate in a commercial filtration application for several months without the formation of pin-holes.
U.S. Pat. No. 5,472,607 to Mailvaganam Mahendran et al discloses a hollow fiber semipermeable membrane in which a tubular macroporous support is superficially coated on its outer surface with a thin film of polymer, most preferably of polyvinylidene difluoride. The tubular braid is flaccid but other details of the structure of the braid are not specified. For example, the effect of characteristics of the material forming the braid were not known; nor was the effect of a cross-section which was not truly circular, i.e. having xe2x80x9ccylindricityxe2x80x9d substantially less than 1.0. The term xe2x80x9ccylindricityxe2x80x9d (sometimes referred to as xe2x80x9croundnessxe2x80x9d) refers to how perfectly the circular cross-section of the tubular support matches the geometry of a true circle drawn to correspond to the mean diameter of the braid, a perfect match being 1.0. It was therefore not known at that time, how critical the physical characteristics of a preferred braid were to the performance of a hollow fiber membrane using the braid.
In commercially available braid, made with conventional braiding equipment from commercially available yarn, there were numerous xe2x80x9cbreaksxe2x80x9d in the fiber; also, accumulation of clumps of broken filaments, referred to as xe2x80x9cfuzzxe2x80x9d, braided into the cylindrical wall of the braid, resulted in weak spots in the polymer film coated onto the surface; and broken filaments, referred to as xe2x80x9cwhiskersxe2x80x9d, protruding from the surface of the tubular braid, resulted in too-thick domains of polymer which were concentrated around the whiskers; and, when the domain was not too-thick, whiskers have a proclivity to initiate pin-holes.
Further, if the open weave of the braid provided either too high or too low a braid porosity as measured by resistance to air flow, the fiber membrane formed was unusable in a commercial installation. Too open a weave resulted in the braid being embedded, that is, enclosed by and firmly fixed in the polymer which also infiltrates into the bore of the braid; thus, too open a weave results in greatly reduced permeability. Too tight a weave results in the polymer not being anchored sufficiently well on the surface; this increases the likelihood that, in service, the polymer film will be peeled from the braid. When operating flux was excellent, portions of the polymer film were sometimes found to have been peeled away when the fibers were backwashed with clean water or other fluid medium, whether water or permeate, under pressure; or portions of the film were xe2x80x9cblown offxe2x80x9d the surface of the fibers when their lumens were pulsed with air under pressure. Even with the best braid produced under controlled conditions, shrinkage during usage in an aqueous medium varied unpredictably. This resulted in taut fibers which were prematurely fouled because they were unable to move sufficiently to stay clean or rub against each other. If too taut, the fibers are broken before they are fouled, or torn from potting resin in the header. Particularly because it is essential for best performance, and to shed contaminants from the surfaces of the hollow fiber membranes, that a skein of fibers operate with xe2x80x9cslackxe2x80x9d fibers, the structure of the braid needs to survive repetitive twisting, and it was not known what physical characteristic(s) of the braid was conducive to such survival. A cylindricity less than 0.8 resulted in a polymer film with unacceptable variations in thickness resulting in non-uniform flux and zones which were too easily fouled.
The goal to achieve great strength led one to choose a high strength yarn, e.g. of glass, aramid or other high modulus material, to benefit from its high strength and stability. For example, a braid woven from glass multifilaments, has insignificant maximum extension at break, less than about 5%, and is essentially non-shrinkable. However, in practice, braided fibers woven with such stable high modulus yarns are not desirable as they provide inadequate adherence of film to the surface of the braid, attributed to the negligible moisture regain of such fibers, and when wet, the braid is too fragile for prolonged service. In particular, it was not known that a certain range of moisture regain in the material of the braid was essential for optimum operation of hollow fiber membranes coated with a hydrophilic polymer film and operated in an aqueous or alcoholic environment. The moisture regain is the percentage of moisture in a textile material brought into equilibrium with a standard atmosphere after partial drying, calculated as a percentage of the moisture-free weight.
The goal to anchor the polymer film non-removably, and to achieve a high xe2x80x9cbubble pointxe2x80x9d in a membrane with no defects (such as pin-holes) was not identified in the ""607 disclosure because the factors which affected the goal were not known. The xe2x80x9cbubble pointxe2x80x9d refers to the pressure under which a stream of air escapes through the largest pore in a wall of a defect-free membrane which has desirable flux. Further, the importance of stability of the structure of the braid during operation, particularly the effect of shrinkage, was not known.
It has been discovered that certain physical characteristics of a tubular braid are critical to the formation of a desirable hollow fiber MF or UF, that is, liquid-separation membrane which is stable and strong, yet has an essentially trouble-free useful life and an acceptable, desirably high, permeability.
It is therefore a general object of this invention to provide a tubular braid support for an asymmetric membrane, woven from yarn made with synthetic resinous filaments essentially insoluble in the solvent in which the membrane-forming polymer is dissolved, the braid having a stable heat-pre-shrunk length which is in the range from about 1% to 20% less than its unshrunk length, preferably so that, irrespective of the material forming the fibers, when the pre-shrunk braid is stretched longitudinally, it has xe2x80x9cgivexe2x80x9d, that is, the extension at break is at least 10%, preferably in the range from 10% to 30%, and more preferably about 20%.
It is a specific object of this invention to provide a heat-pre-shrunk tubular braid made with specified patterns, using carriers carrying yarn having defined number of filaments, ends, denier, and picks, under conditions which control the porosity (measured as permeability to air) of the braid, such controlled porosity serving to anchor a polymer film non-removably on the surface of the tubular braid.
It is another specific object of this invention to provide, in a flexible macroporous tubular braid support for an outside-in hollow fiber asymmetric membrane having a tubular film of synthetic resinous material supported on the outer circumferential surface of the braid without the support being embedded in a thin film having a wall thickness of less than 0.2 mm, the improvement comprising, 16 to 60 separate yarns, each on its own carrier, each yarn being multifilament 150 to 500 denier (g/9000 meters) yarn, each multifilament being made with from 25 to 750 filaments, each filament being from 0.5 to 7 denier. From 1 to 3 multifilament ends constitute a yarn, and the individual ends are most preferably not plied together, but lie linearly adjacent to each other until taken up in the xe2x80x9cfellxe2x80x9d of the braid being woven. The braid being woven has from 30 to 45 picks (crosses/inch). The higher the denier of the filaments, the fewer the filaments used, but the braid wall thickness is maintained in the range from about 0.2 mm but less than three times the diameter of the yarn from which the braid is woven, preferably less than 1.0 mm. The air permeability of the braid of synthetic resinous yarn is in the range from about 1 to 10 cc/sec/cm2 at a differential pressure of 1.378 kPa (0.2 psi); and the moisture regain is in the range from about 0.2% to 7% by weight (wt). The finished fiber membrane is coated with a thin polymer film having a thickness in the range from 0.05 mm to 0.3 mm, most preferably less than 0.1 mm thick. The film has an annular peripheral barrier layer or xe2x80x9cskinxe2x80x9d circumferentially integral with successive microporous layers in the film, each layer contiguous with a preceding layer, the layers including an outer annular layer, an intermediate transport layer, and an annular inner layer.